nfs4: strengthen error check to avoid unexpected result

The variable error is ssize_t, which is signed and will
cast to unsigned when comapre with variable size, so add
a check to avoid unexpected result in case of negative
value of error.
Signed-off-by: Chengguang Xu <cgxu519@mykernel.net>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

fs/nfs/nfs4proc.c

@@ -7440,7 +7440,7 @@ nfs4_listxattr_nfs4_label(struct inode *inode, char *list, size_t list_len)
 
 	if (nfs_server_capable(inode, NFS_CAP_SECURITY_LABEL)) {
 		len = security_inode_listsecurity(inode, list, list_len);
-		if (list_len && len > list_len)
+		if (len >= 0 && list_len && len > list_len)
 			return -ERANGE;
 	}
 	return len;